The present invention relates generally to electrical power distribution equipment. More particularly, the invention relates to surge arresters. Still more particularly, the invention relates to surge arresters employing polymeric weathersheds.
Under normal operating conditions, electrical transmission and distribution equipment is subject to voltages within a relatively narrow range. Due to lightning strikes, switching surges or other system disturbances, portions of the electrical network may experience momentary or transient voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. Left unprotected, critical and costly equipment such as transformers, switching apparatus, computer equipment, and electrical machinery may be damaged or destroyed by such over-voltages and the resultant current surges. Accordingly, it is routine practice to protect such apparatus from dangerous over-voltages through the use of surge arresters.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert the over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When caused to operate, a surge arrester forms a current path to ground having a very low impedance relative to the impedance of the equipment that it is protecting. In this way, current surges which would otherwise be conducted through the equipment are instead diverted through the arrester to ground.
Conventional surge arresters typically include an elongate outer housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the housing for connecting the arrester between a line-potential conductor and ground, and an array of electrical components in the housing that form a series path between the terminals. These components typically include a stack of voltage dependent, nonlinear resistive elements. These nonlinear resistors or "varistors" are characterized by having a relatively high resistance at the normal steady-state voltage and a much lower resistance when the arrester is subjected to transient over-voltages. Depending on the type of arrester, it may also include one or more electrodes, heat sinks or spark gap assemblies housed within the insulative housing and electrically in series with the varistors.
To ensure proper operation of the arrester, the varistors and other internal components must be isolated from moisture and environmental pollutants. The arrester housing serves to seal the components from the ambient environment. Additionally, most surge arrester housings include "skirts" or "weathersheds" spaced apart along the length of the housing. An arrester, once installed outdoors, will be exposed to contaminants or environmental pollutants that are deposited on the housing surface by rain, wind and other conditions. These contaminants, over time, may build up to such a degree that they form a path for current. Such buildup effectively reduces the distance between energized or line-potential components and ground. In this manner, the surface resistivity of the arrester housing will decrease to a point where flashover may occur and a short circuit result. Accordingly, weathersheds have traditionally been included on an arrester housing to extend or lengthen the housing surface and increase the effective distance between the energized arrester terminal and ground. Additionally, weathersheds have been designed to enhance the ability of the arrester to resist or to minimize the degree to which dust and environmental contaminants may build up on the housing's outer surface. Such designs have included varying the radii of adjacent sheds, using particularly designed materials that resist the effects of contamination, and by varying the number and size of the sheds on the housing.
Surge arrester housings made of porcelain were once the industry standard. Unfortunately, such arrester housings were fragile and frequently were the subject of vandalism. Additionally, the porcelain housing was heavy, requiring a substantial support means to mount the arrester. Furthermore, when a porcelain housed arrester failed, it was not uncommon for the housing to explode, sending porcelain fragments at high velocities in all directions. Such failures presented the obvious potential for danger to personnel and damage to equipment.
Presently, at least in distribution class surge arresters, a polymeric housing has become a standard feature. A polymeric housing is less expensive to manufacture, is nonfragmenting and is less susceptible to damage during shipment, installation and use compared to prior art porcelain housings. Additionally, a polymeric housing is substantially lighter, allowing simpler and less costly installation.
The polymeric arrester housing is typically molded of silicone rubber or another elastomeric material. The housing includes a central core and radiating sheds or skirts which are molded integrally with the central core. The central core includes an internal bore or chamber that is substantially the same diameter as the varistors and other arrester components to be housed therein. Where a particular shape or orientation of the sheds is desired, the mold for the housing is manufactured so as to provide that desired configuration.
Present molding techniques effectively limit the configuration and arrangement of sheds on a polymeric arrester housing. Further, because of limitations in the molding process, manufacturing housings with certain weathershed orientations is costly and difficult. Also, the present methods of obtaining a good bond between the inside surface of the housing and the internal components is expensive and generates a substantial amount of scrap material.
Accordingly, there remains a need in the art for a polymeric arrester housing having an enhanced weathershed design that will resist buildup of environmental pollutants and, at the same time, is relatively simple to manufacture using conventional molding techniques. It would further be advantageous if the housing provided a superior bond between the inside surface of the housing and the internal electrical components. Given the present cost of silicone rubber and other elastomeric materials known to be employed in arrester housings, it would be further advantageous if the weathershed could be manufactured using less material than presently employed for similar housings.